Apparatus and method for testing static and kinetic frictional coefficients of a sheet material

ABSTRACT

A horizontal plane frictional coefficient testing apparatus and methods are provided which include a substantially horizontal planar platen having two spaced, parallel movable rails. The rails are actuated by a motor adapted for reversibly raising and lowering the rails relative to the platen. A sled of predetermined weight is also provided having a central body and two pairs of outwardly extending axles, with each pair of axles being of equal length and formed with a wheel at the end thereof. The wheels of the sled are formed for engagement with the rails and engagement of the wheels with the rails locates the sled generally parallel to the rails. In operation, a sample of sheet material is fixed to the sled and the sled is placed on the rails, with the rails being in a raised position. Subsequently, the rails are lowered to gently place the sled onto the platen or a test medium fixed to the platen. A drive arm is included with the apparatus for pulling the sled relative to the platen at various rates of translation. A load sensor mounted on the drive arm measures in real time the force applied to the sled during the course of the test. The data collected by the load sensor is transmitted to a central processing unit which calculates the coefficients of friction. A timing mechanism may be provided to monitor the dwell time of the sled on the platen prior to initiation of the tests. Also, a semi-rigid coupling may be used to connect the sled to the drive arm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to apparatus and methods for testing static andkinetic frictional coefficients of sheet material and, moreparticularly, an automated horizontal plane apparatus for testing staticand kinetic frictional coefficients of sheet material and methodsrelated thereto.

2. Description of the Prior Art

It can be appreciated that the determination of frictionalcharacteristics of sheet materials, such as paper, film, rubber,plastics, wood, linoleum, and coatings, is crucial to evaluate theworkability of the sheet materials in processes and by the end consumer.For example, paper photocopying machines are formed with rollers andpaper feeding mechanisms which are designed to cooperatively feed sheetsof paper having frictional coefficients within certain predeterminedranges. If the frictional coefficients are outside of these ranges, thephotocopying machine may fail to feed the paper sheets or may feed thepaper sheets irregularly. As a second example, linoleum sheets used asflooring must be formed with certain frictional characteristics whichwill allow a person to comfortably and safely walk thereupon.

Horizontal plane testing apparatus have been developed in the prior artto test static and kinetic frictional coefficients of sheet materials.The prior art devices include a horizontal platen with a clampingmechanism for rigidly fixing a sample of the sheet material to theplaten. Typically, frictional coefficient testing apparatus are used totest the frictional coefficients between two samples of the same sheetmaterial, although the frictional coefficients between two differentsheet materials may also be tested. A second sample of sheet material issecured to a planar face of a sled. The sled is a generallyparallelepiped shaped block having a predetermined weight. In the priorart, the sled is manually placed on the platen, with the two samples ofsheet material being in face-to-face engagement, at one end of the firstsample of sheet material. The sled is driven across at least a portionof the length of the first sample of sheet material at a constant speedalong a single linear axis, and a load cell is coupled to the sled tomeasure the force required at all times to drive the sled across thelength of the sheet material. A data acquisition system receives datameasured by the load cell and calculates static and kinetic coefficientsof friction for the interface of the two samples of sheet material. Thestatic coefficient is an indicator of the degree of force required toinitially move or "slip" the sled from a stationary position in a singlecoordinate direction, and, thus, it is calculated during the initialportion of the test, with the sled resting motionless on the platen. Thevalue of the static coefficient is obtained by dividing the amount offorce required to initially move the sled by the weight of the sled. Incontrast, the kinetic coefficient is an indicator of the degree of forcerequired to continue movement of the sled in a single coordinatedirection, with the sled already being in motion. During the test, thekinetic coefficient may be determined once the sled is in motion and iscalculated by dividing the amount of force required to continue movementof the sled by the weight of the sled.

An example of a prior art horizontal plane static and kinetic frictioncoefficient testing apparatus is manufactured and sold under thetradename "MONITOR/SLIP & FRICTION™" by Testing Machines, Inc. ofIslandia, N.Y.

The prior art apparatus suffers from several drawbacks. First, themanual placement of the sled onto the platen results in testing errorsand lack of testing repeatability due to varying forces being applied tothe interface of the tested samples of sheet materials by an operator oroperators of the apparatus. A slight difference in the degree of forceapplied in placing the sled onto the platen, greater or smaller, maytranslate to respectively greater or less meshing of the engagedsurfaces of the samples, with corresponding increases or decreases inthe measured values of the static frictional characteristics of thesamples. Consequently, errors are created in measured data. Furthermore,even with an apparatus accurately measuring coefficients, the magnitudeof the frictional coefficients will vary from test to test of the samesamples due to the differences in pressures applied in placing the sledfor each test. As such, the determination of the actual coefficients ofa sheet material is quite difficult, requiring repeated testing of thesame sheet material and intense scrutiny of such test results. Thus,there is a need for a testing apparatus which can measure static andkinetic frictional coefficients of a sheet material with repeatedaccuracy. It should be noted that absolute test repeatability is notrequired where exact test results are duplicated; however, test resultsshould repeat within a statistically acceptable range. As used herein,"repeatable" and "repeatability", respectively, refer to test resultsobtained, and the ability to obtain test results, respectively, from thesame samples which fall within statistically acceptable ranges of eachother.

Second, manual placement of the sled onto the platen may result inmisalignment of the sled relative to the platen. Since the force used todetermine the static and kinetic frictional coefficients is linearlyapplied to the sled along a single axis, to obtain repeatable testmeasurements of a static frictional coefficient, the sled must bealigned in the same manner relative to the linear axis of the force foreach test. Manual placement of the sled often results in misalignment,with attendant distorted results and lack of test repeatability. Also,the linear force is typically applied through a single point couplingwith the sled. If the sled is not fully coaxially aligned with the axisof the linear force, the coupling acts as a fulcrum about which the sledrotates slightly into alignment with the linear axis during initialmovement. The rotational motion of the sled relative to the platengenerates resistive frictional forces in two coordinate directions, onecoordinate direction being parallel to the linear axis and the secondcoordinate axis being perpendicular to the linear axis. Since thecoefficients of friction are determined relative to the degree of forcerequired to move the sled in one coordinate direction, the additionalfrictional forces generated in the second coordinate direction distortthe test results. It can be appreciated that manual placement of thesled onto the platen often results in misalignment of the sled relativeto the platen and subsequent slight rotational movement of the sled,resulting in a lack of repeatability in tests and inaccurate frictionalcoefficient measurements.

Third, a mechanism to control the dwell time of the sled on the platenrelative to the actual start of the test is not present in the prior artapparatus. Once the sled is manually placed on the platen, the prior artapparatus must be manually actuated. However, the dwell time, the amountof time the sled rests on the platen prior to initiation of a test,affects the outcome of the test. With certain materials, "blocking",which is additional resistance to movement beyond friction, may occur asa result of molecular adherence between the tested materials caused byexcessive dwell time. During such tests, forces applied to the sled mustovercome not only friction, but also molecular adherence, to move thesled and, consequently, test results will be distorted. If there is aninadequate amount of dwell time, air trapped between the two samples ofsheet material may not have an opportunity to evacuate, and a layer ofair may be entrapped between the two samples during testing. The layerof air will act as a cushion and lessen the frictional forces generatedbetween the two samples. Attempts have been made in the prior art tomanually monitor dwell time to achieve repeatability of test results. Aswith the manual placement of the sled onto the platen, manualmeasurement of the dwell time is affected by human error, resulting in afailure to achieve testing repeatability.

Fourth, prior art horizontal plane frictional coefficient testingapparatus are configured to drive the sled during the course of the testat a single speed from initiation to completion. Each test run includestwo portions, an initial static coefficient test portion and asubsequent kinetic coefficient test portion. The simultaneous end of thefirst portion of the test and the beginning of the second portion of thetest is an instantaneous point in time at which the sled initially movesrelative to the platen. The use of a constant test speed is appropriatefor determining the kinetic coefficient of friction, however, theconstant rate may lead to inaccuracies in determining the staticcoefficient of friction. With the application of the constant rate ofspeed from initiation of the test, a jolt or shock is applied to thesled initially which may distort force readings collected during thestatic coefficient phase of the test. Also, with a constant speed test,the force is applied to the sled with a constant rate. Since the captureof data relating to the instantaneous point of initial movement of thesled is essential to determining the static coefficient of friction, aslow speed, and thus a slow rate of applying force, is preferred toensure accurate measurement of past data. However, with the amount oftime to perform a test also being a factor, apparatuses are typicallyconfigured to run at a constant speed which generates a rate of forceapplication greater than the aforementioned preferred rate. Repeatedtest runs at a constant rate slow enough to ensure accurate staticfrictional coefficient measurements may be overall too time-consuming ina commercial environment.

Fifth, prior art apparatus utilize rigid coupling systems which are notadaptable to the two portions of the testing procedure. A rigid couplingis preferred for the kinetic coefficient testing portion which allowsconstant transmission of the driving force to the sled, whereas, incontrast, some elasticity is preferred in the coupling for the staticcoefficient portion of the test. An elastic coupling would allow a loadto be applied to the sled gradually, rather than abruptly.

A sixth drawback in the prior art is the rearward translation of thesled relative to the platen due to manual placement and/or removal ofthe sled from the platen. As described above, prior to a test run in theprior art, the sled is manually placed onto the platen. However, oftenthe sled is manually placed on the platen with subsequent movement ofthe sled in an opposite direction from the direction in which the sledis to be translated during the test run. As a result, the frictionbetween the interengaging faces of the samples of sheet material causesthe grain of the surfaces of the samples to be urged into the oppositedirection. Since several tests are often performed on the same samplesof sheet material, the disturbed grain of the sample surfaces underminesrepeatability in test results. Furthermore, manual removal of the sledfrom the platen after a test run may also cause rearward movement of thesled relative to the platen and further disruption of the grain of thesample surfaces. To enhance test repeatability, "back sliding" of thesled relative to the platen should be avoided.

It is an object of the subject invention to provide a horizontal planefrictional coefficient testing apparatus which overcomes theshortcomings of the prior art devices and measures static and kineticfrictional coefficients with repeatability.

Also, it is an object of the subject invention to provide a horizontalplane frictional coefficient testing apparatus which provides forautomated reversible lifting and lowering of a sled.

It is also an object of the subject invention to provide a frictionalcoefficient testing apparatus which ensures proper alignment of the sledrelative to the platen of the apparatus.

It is a further object of the subject invention to provide a horizontalplane frictional coefficient testing apparatus which operates at morethan one speed during a single test and is provided with a semi-rigidcoupling for the sled which reacts to the operating speed of theapparatus.

Yet another object of the subject invention is to provide an apparatuswhich monitors dwell time of the sled on the platen and initiates testruns at the lapse of predetermined dwell times.

It is a further object of the subject invention to provide a horizontalplane frictional coefficient testing apparatus which preventstranslation of the sled before and after a test run in a directionopposite the testing direction.

SUMMARY OF THE INVENTION

The above-stated objects are met by a new and improved horizontal planefrictional coefficient testing apparatus. The apparatus includes asubstantially planar horizontal platen formed with two spaced, parallelmovable rails and a clamping mechanism. The rails are actuated by amotor adapted for reversibly raising and lowering the rails relative tothe platen. A sled is provided having a central body, resembling thesleds of the prior art, and two pairs of outwardly extending axles, withone pair of the axles extending from each side of the sled being ofequal length and with each axle having a wheel rotatably mounted to theend thereof. A drive arm is included with the apparatus for pulling thesled during a test run, which includes two phases: a static coefficienttesting phase and a kinetic coefficient testing phase.

The wheels of the sled are formed for engagement with the rails. Withthe wheels engaging the rails, the sides of the sled are generallyparallel to the rails. In operation, a sample of a sheet material isfixed to the sled. The frictional characteristics of the sample may betested with respect to a test medium, which could be a second sample ofsheet material fixed to the platen by the clamping mechanism or aliquid. Additionally, the frictional characteristics of the sample maybe tested with respect to the bare platen, which acts as a test surface.The sled is placed on the rails, with the rails being in a raisedposition, and, subsequently, the rails are lowered to gently place thesled onto the sample clamped to the platen, in contact with the liquidor onto the bare platen. Automated lowering of the sled onto thesample/platen ensures repeated placement of the sled with the samedegree of placement force, and the engagement of the wheels and railsensures repeated proper alignment of the sled relative to the platen. Atiming mechanism may be provided to measure dwell time from the momentthe rails are lowered with the sled being placed on the sample/platenand to generate a signal to initiate the test run upon lapse of thepredetermined dwell time.

Once the test run is initiated, a drive arm applies a driving force tothe sled. The drive arm is coupled to the sled through a semi-rigidcoupling. Specifically, the sled is formed with a downwardly directedcoupling member formed to register with an inner channel of a tubularviscoelastic bushing disposed in an aperture in the drive arm. At theinitiation of the test run, the coupling member is centrally located inthe channel of the viscoelastic bushing, with no contact between thecoupling member and the bushing. Upon actuation of the drive arm, thestatic coefficient testing phase begins, and the drive arm moves at arelatively slow constant rate. With movement of the drive arm, thecoupling member comes into pressing contact with the bushing. Theviscoelastic material of the bushing is selected such that slightdeformation of the bushing is achieved due to the pressing contact ofthe coupling member thereon. As a result of the deformation of thebushing, although the drive arm is moving at a constant rate, force isgradually applied to the sled. A load sensor is mounted on the drive armto collect real time force data with respect to the sled. The real timedata collected by the load sensor is transmitted to a central processingunit for data evaluation. The length of time of the static coefficienttesting phase of the test run is predetermined. The central processingunit is configured to initiate the kinetic coefficient testing phase ofthe test run, upon lapse of the predetermined length of time. In thekinetic coefficient testing phase of the test run, the speed of thedrive arm is increased and maintained at the increased rate to the endof the test run. Real time data is continuously collected by the loadsensor and evaluated by the central processing unit.

In this manner, the apparatus of the subject invention allows the staticfrictional coefficient phase of the test to operate at a relatively slowrate and the kinetic frictional coefficient phase of the test to operateat a faster rate, thereby allowing for proper data gathering within anacceptable test-taking time frame.

The load sensor measures force values over a number of positions as thesled translates the length of the test run. The central processing unitcollects the measured data and calculates the frictional coefficients.When the sled approaches the end of the test run, the central processingunit actuates the reversible elevating mechanism of the rails, and therails are raised into the upper position. The rails are caused toelevate with the sled moving forward, such that the sled continues tomove forward with the rails being raised, and "back sliding" of the sledis altogether avoided.

The viscoelastic material of the bushing is selected such that slightdeformation of the bushing is achieved during initiation of the testrun, with the driving force being initially applied, and, once slightlydeformed, the bushing is sufficiently rigid to prevent furtherdeformation. Therefore, the viscoelastic bushing provides elasticityduring the static frictional coefficient testing portion of the test runand substantial rigidity during the kinetic frictional coefficientportion of the test run.

In an alternative embodiment, an anti-skid arm may be hingedly connectedto the drive arm. The anti-skid arm can be formed to cooperate with thesled to ensure proper alignment of the sled with the coupling member ofthe sled being centrally located in the channel of the viscoelasticbushing at the initiation of a test run. Also, the anti-skid arm caninclude guide pins for limiting misalignment of the sled during the testrun.

Additionally, the platen may be provided with an inset heating mechanismwhich can raise the temperature of the platen and any test medium whichmay be disposed thereon. With the ability to heat the platen and thetest medium, the apparatus of the subject invention does not only allowan operator to determine the effect of heat on the frictionalcharacteristics of the sample of sheet material, but also the operatorcan generally observe the effect of heat on the sample.

These and other features of the subject invention will be betterunderstood through an examination of the following detailed descriptionand accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of the first embodiment of the subjectinvention, parts of which are schematically illustrated.

FIG. 2 is a side elevational view of the first embodiment of the subjectinvention with the rails and the sled being in a raised position.

FIG. 3 is a side elevational view of the first embodiment of the subjectinvention with the rails being in a lowered position and the sledresting on the platen.

FIGS. 4a-4c are partial views of three separate embodiments of thewheels of the sled of the subject invention.

FIG. 5 is a side elevational view of the embodiment of the subjectinvention being provided with an anti-skid arm, wherein the rails andthe sled are in a raised position.

FIG. 6 is a side elevational view of the embodiment of the subjectinvention being provided with an anti-skid arm, wherein the rails are ina lowered position and the sled is resting on the platen.

FIG. 7 is a partial rear elevational view of the embodiment of thesubject invention being provided with an anti-skid arm, wherein thebridge of the sled is interposed between the guide pins of the anti-skidarm.

FIG. 8 is a partial side elevational view of the embodiment of thesubject invention being provided with a pan for accommodating a liquid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring generally to the FIGURES, an apparatus 10 is provided fortesting the static and kinetic frictional coefficients of the interfaceof a test medium or a test surface and a sample of sheet material 14. Inthe first embodiment of the invention, the apparatus 10 is for testingthe static and kinetic frictional coefficients of the interface betweentwo samples of sheet material 12, 14. The samples 12, 14 may be of thesame sheet material or, alternatively, of two different sheet materials.The apparatus 10 generally comprises a platen 16, a sled 18, a drive arm20, a load sensor 22, and a central processing unit 24.

The platen 16 is generally planar and horizontal. The platen 16 must behorizontal so that the weight of the sled 18 generates a force normal tothe platen 16. In calculating the frictional coefficients, it is assumedthat the weight of the sled 18 is a normal force acting on the platen16, and any misalignment of the platen 16 from a horizontal plane willdistort the test results. The platen 16 is preferably formed with arectangular shape. At one end of the platen 16, a clamp 26 is providedfor rigidly securing the first sample of sheet material 12 to the platen16. The clamp 26 may be of any conventional design. The FIGURESrepresent an exemplary embodiment of the clamp 26 wherein a clamp body28 is provided which is pivotally mounted to pedestals 30, 32 by pivots36. The clamp 26 further includes a biasing means (not shown) for urgingedge 38 of the clamp body 28 into abutting contact with the platen 16.The edge 38 of the clamp body 28 is separated from the platen 16 byapplying a pulling force to handle 40, which extends from the clamp body28. The first sample of sheet material 12 is fixed to the platen 16 withthe first sample 12 being interposed between the edge 38 of the clampbody 28 and the platen 16 and the edge 38 being biased toward the platen16.

The platen 16 is also formed with a pair of spaced, parallel elongatedslots 42, 44. A single rail 46, 48 is movably disposed in each of theelongated slots 42, 44 respectively. Reversible elevating and loweringmeans 50 is mechanically coupled to the rails 46, 48 to reversibly raiseand lower the rails 46, 48 relative to the platen 16. The rails 46, 48must be raised and lowered simultaneously.

The sled 18 is of a known weight and formed with a generallyparallelepiped shaped central body 52 and a plurality of axles 54. Asshown in FIG. 1, each of the axles 54 extending from one side 56, 58 ofthe sled 18 is of the same length. Referring to FIG. 1, the axles 54extending from the side 56 of the sled are each formed with the length"x", whereas, the axles 54 extending from the side 58 are each formedwith the length "y". All of the axles 54 may be formed with the samelength, where length "x" equals length "y". The length or lengths of theaxles 54 must be chosen such that the bridge 70 of the sled 18,described below, is coaxially aligned with the longitudinal axis alongwhich force is applied to the sled 18 by the drive arm 20. Although theFIGURES show four of the axles 54 extending from the sled 18, any numberof the axles 54 may be used with the sled 18. Preferably, an equalnumber of the axles 54 extend from the side 56 of the sled 18 as fromthe side 58.

A wheel 60 is mounted to the end of each of the axles 54 and the wheels60 are preferably rotatably mounted to the axles 54. The wheels 60 arepreferably formed for rolling engagement with the rails 46, 48. As shownin FIGS. 4a-4c, each of the wheels 60 is formed with a central hub 62and at least one circumferentially extending rim 64. The rims 64 of thewheels 60 collectively ensure that the hubs 62 maintain engagement withthe respective rails 46, 48. Also, due to partial face-to-face contactof the rims 64 and the rails 46, 48, the rims 64 ensure repeatablealignment of the wheels 60 relative to the respective rails 46, 48,thus, ensuring repeatable alignment of the sled 18 relative to theplaten 16. With the axles 54 being of the same length from each of thesides 56, 58 of the sled 18, the axles 54 will cause the sides 56, 58 ofthe sled 18 to be substantially parallel to the rails 46, 48. In thismanner, the sled 18 may be repeatedly aligned in the same mannerrelative to the platen 16 so that under a driving force the sled 18translates along a linear path and rotational movement, such as thatfound with prior art devices, is avoided.

The central body 52 of the sled 18 is also formed with a top surface 66and a bottom surface 68. A bridge 70 is secured to the top surface 66from which a coupling member 72 extends downwardly. The bottom surface68 is substantially planar and formed to accommodate the second sampleof sheet material 14. The axles 54 are located on the respective sides56, 58 of the sled 18 such that with the sled 18 being disposed on agenerally planar surface, such as the platen 16, the wheels 60 do notcome into contact with the planar surface. Also, the axles 54 arelocated on the respective sides 56, 58 of the sled 18 to align thebottom surface 68 of the sled 18 in a generally parallel relationship tothe platen 16 with the wheels 60 engaging the rails 46, 48.

The drive arm 20 is mechanically coupled to a drive mechanism 74. Thedrive mechanism 74 is configured to cause linear translation of thedrive arm 20 at various rates of translation. The load sensor 22 ismounted to the end of the drive arm 20 with coupling 76 extendingtherefrom. The coupling 76 is formed with a coupling aperture 78 whichhouses a viscoelastic bushing 80 having an inner channel 82 dimensionedslightly larger than the coupling member 72 of the sled 18. The pathalong which the coupling aperture 78 travels during a test run definesthe longitudinal axis along which force is applied to the sled 18. Asdescribed above, the bridge 70 is located to be generally coaxial withthis axis. The coupling member 72 preferably is located on a point alongthe longitudinal center line of the bridge 70. Although not shown,alternatively, the coupling 76 may be formed with the coupling member72, and the bridge 70 may be formed with the coupling aperture 78 havingthe viscoelastic bushing 80 being disposed therein. However, with thisalternative embodiment, the coupling member 72 must extend away from theplaten 16 to allow simultaneous placement of the sled 18 onto the rails46, 48 and telescopic mating of the inner channel 82 of the bushing 80with the coupling member 72 in a spaced relationship.

With the sled 18 being coupled to the drive arm 20 and translatingrelative to the platen 16 as described below, the load sensor 22collects real time data of forces transmitted through the coupling 76required to cause translation of the sled 18 relative to the platen 16.The real time data collected by the load sensor 22 is transmittedthrough connector 84 to the central processing unit 24 for real timeevaluation. The weight of the sled 18 is input into the centralprocessing unit 24 and the respective static and kinetic coefficients offriction are calculated using methods well known by those skilled in theart. The central processing unit 24 is also connected to the reversibleelevating and lowering means 50 via connector 86 and the drive mechanism74 via the connector 88. The central processing unit 24 is configured tocontrol the reversible elevating and lowering means 50 and the drivemechanism 74.

In operation, the first sample of sheet material 12 is secured to theplaten 16 by the clamp 26. The second sample of sheet material 14 issecured to the bottom surface 68 of the sled 18 using any conventionalmethod known by those skilled in the art, such as clamping (not shown)or adhering (not shown). With the rails 46, 48 being in a raisedposition, as shown in FIG. 2, the sled 18, being of known weight, isplaced onto the apparatus 10 with the wheels 60 engaging the rails 46,48. The rims 64 of the wheels 60 respectively come into partialface-to-face engagement with the rails 46, 48. Simultaneously, thecoupling member 72 is aligned to register with the inner channel 82 ofthe semi-rigid viscoelastic bushing 80. In this position, the secondsample of sheet material 14 is maintained in a spaced, parallelrelationship relative to the first sample of sheet material 12 fixed tothe platen 16. Subsequently, the central processing unit 24 isactivated, which actuates the reversible elevating and lowering means 50and the rails 46, 48 are lowered relative to the platen 16. As the rails46, 48 are lowered, the second sample of sheet material 14 comes intoface-to-face engagement with the first sample of sheet material 12 fixedto the platen 16, and the coupling member 72 telescopically mates withthe inner channel 82 of the semi-rigid viscoelastic bushing 80. Thecoupling member 72 is disposed to be in the inner channel 82 such thatthere is no contact between the coupling member 72 and the semi-rigidviscoelastic bushing 80.

As shown in FIG. 3, with the rails 46, 48 being in a completely loweredposition, the sled 18 rests on the platen 16 with the axles 54 and thewheels 60 being spaced from the platen 16. Thereafter, the centralprocessing unit 24 actuates the drive mechanism 74 to cause the drivearm 20 to translate relative to the platen 16. A timing means 90 may beprovided to measure the dwell time of the sled 18 on the platen 16. Thetiming means 90 can be connected to the central processing unit 24through a connector 92 and adapted to receive a signal from the centralprocessing unit 24 indicating the placement of the sled 18 onto theplaten 16, and transmitting a signal to the central processing unit 24after the lapse of a predetermined amount of dwell time. Alternatively,the timing means 90 may be formed integrally with the central processingunit 24, or, the central processing unit 24 may perform the functions ofthe timing means 90, thereby, obviating the need for the separate timingmeans 90.

As the drive arm 20 translates, force is transmitted to the sled 18through the coupling 76. Initially, the drive mechanism 74 causes thedrive arm 20 to translate at a slow constant rate. The translation ofthe drive arm 20 causes the coupling member 72 to press against theviscoelastic bushing 80. The viscoelastic material of the bushing 80 ischosen such that, under slight pressure, the viscoelastic materialcompresses relatively easily. Consequently, the initial forcetransmitted to the bushing 80 by the coupling member 72 is partiallyabsorbed by compression of the viscoelastic material of the bushing 80.As greater force is applied to the bushing 80 by the coupling member 72,the viscoelastic material resists further compression and, at the pointof contact between the coupling member 72 and the bushing 80, eventuallybecomes substantially rigid. As a result, although the drive arm 20translates at a slow constant rate, the compression of the bushing 80advantageously allows force to be initially applied to the couplingmember 72 at a gradually increasing rate. The compression of the bushing80 causes force to be absorbed, and, as the bushing 80 becomes fullycompressed, force is directly transmitted from the drive arm 20 to thesled 18, via the coupling member 72.

The drive arm 20 will linearly translate at the slow constant rate for apredetermined period of time, which is the static coefficient testingphase of the test run. The static coefficient testing phase of the testrun requires relatively little time. For example, with the drive arm 20translating at a rate of 0.4 inches/minute, the static testing phase ofthe test run may only require several milliseconds, during which thesled 18 "slips" into motion. It can be appreciated that real timeevaluation of the static coefficient of friction is not practicablyfeasible. Thus, the time period for the static coefficient testing phaseis predetermined by the user of the apparatus 10. If the staticcoefficient of friction of the interface of the samples 12, 14 iscompletely unknown, the user of the apparatus 10 may iteratively testthe samples 12, 14 using various lengths of time for the staticcoefficient testing phase of each of the runs, until the staticcoefficient of friction is determined with repeatable accuracy. If theapparatus 10 is used in an application where the static coefficient ofthe samples 12, 14 is known, such as with random quality assurancetests, the length of the static coefficient phase of the test run is setto ensure capture of static coefficient test data.

During the static coefficient testing phase of the test run, asdescribed above, force is applied to the sled 18 gradually. As the forceapplied to the sled 18 is slowly increased, the sled 18 is eventuallycaused to linearly translate relative to the fixed first sample of sheetmaterial 12 and the platen 16. The amount of force required to initiallymove the sled 18 is determinative of the static coefficient of frictionof the interface of the two samples of sheet material 12, 14. Data offorce measurements applied to the sled 18 are transmitted from the loadsensor 22 to the central processing unit 24 via the connector 84. Thecentral processing unit 24 evaluates the collected force data anddetermines the point of initial movement of the sled 18 by identifyingthe greatest force applied to the sled 18. The value of the greatestforce is used by the central processing unit 24 to calculate the staticcoefficient of friction. Once the predetermined length of the staticcoefficient testing phase lapses, the kinetic coefficient testing phaseof the test run is initiated, whereby the central processing unit 24transmits a signal to the drive mechanism 74 to increase the rate oftranslation of the drive arm 20 and to constantly maintain the increasedrate of translation of the drive arm 20 to the end of the test run. Theload sensor 22 continues to measure forces applied to the sled 18, andthe collected force data is evaluated by the central processing unit 24to determine the kinetic coefficient of friction.

Due to the elasticity of the viscoelastic material of the bushing 80,the degree of rigidity at the interengagement of the coupling member 72and the bushing 80 is reactive to the different phases of the test run.During initiation of the static coefficient testing phase, the couplingmember 72 presses against the bushing 80. The viscoelastic material ofthe bushing 80 is chosen such that, with initial application of theforce, the viscoelastic material slightly compresses due to pressuregenerated by the coupling member 72 pressing against the bushing 80. Theslight compression of the elastic bushing 80 allows force to beinitially gradually applied to the sled 18 and avoids a suddenapplication of force, which may cause a jolt or shock, to the sled 18and distort test results during the static coefficient testing phase.The compressibility of the viscoelastic material of the bushing 80 islimited such that, once slightly compressed, the viscoelastic materialof the bushing 80 becomes substantially rigid. Thus, during the kineticcoefficient testing phase, the bushing 80, being substantially rigid atthe point of engagement with the coupling member 72, directly transmitsall of the force acting on the drive arm 20 through the coupling 76 tothe coupling member 72 and, in turn, to the sled 18. Consequently, thebushing 80 allows the coupling 76 to be semi-rigid and reactive to theinitial static coefficient testing phase of the test and the subsequentkinetic coefficient testing phase of the test.

The length of the test run can be input into the central processing unit24, as well as the rates of translation of the drive arm 20. As the sled18 approaches the end of the test run, the central processing unit 24transmits a signal via connector 86 to the reversible elevating andlowering means 50 to elevate the rails 46, 48 relative to the platen 16.As the sled 18 completes the test run, the rails 46, 48 engage thewheels 60 and the sled 18 is lifted from the platen 16. As the sled 18is being lifted from the platen 16, the sled 18 continues to move in thetesting direction. In this manner, "back sliding" of the sled 18 in adirection opposite the direction of the test is altogether avoided. Withthe rails 45, 48 being in a raised position, the sled 18 and the drivearm 20 may be reset for a subsequent test run.

A liquid 91 may be interposed between the two samples of sheet material12, 14. For example, the liquid 91 can be a lubricant to determine theeffect of the lubricant on the frictional characteristics of the twosamples of sheet material 12, 14. As shown in FIG. 8, a shallow pan 93may be placed on the platen 16. The pan 93 is formed with a base 95 anda side wall 97 extending from the entire periphery of the base 95. Thebase 95 is dimensioned to accommodate the first sample of sheet material12, and may be either secured to the platen 16 (not shown) or securelyfixed to the platen 16 by the clamp 26. The features and the operationof the apparatus 10 otherwise is exactly as described above.Alternatively, in a second embodiment of the invention, the apparatus 10may be used to measure the frictional characteristics of the interfaceof the sample of sheet material 14 and the liquid 91. To achieve suchresults, only the liquid 91 is provided in the pan 93, and the firstsample of sheet material 12 is not required. In a third embodiment ofthe invention, the apparatus 10 can measure the frictionalcharacteristics of the interface of the sample of sheet material 14 andthe platen 16, which acts as the test surface. Although not shown in theFIGURES, the features and the operation of the apparatus 10 are exactlyas described above, except the sled 18 with the sample of sheet material14 is placed directly onto the bare platen 16.

Referring to FIGS. 5-7, an anti-skid arm 94 may be hingedly mounted tothe drive arm 20 in any of the above-described embodiments. Theanti-skid arm 94 may serve two functions. First, a locator pin 96 mayextend from the underside of the anti-skid arm 94 and be located toregister with a locator aperture 98 formed in the bridge 70 of the sled18 with the anti-skid arm 94 being in a down position. The registrationof the locator pin 96 and the locator aperture 98 ensures properalignment of the sled 18 relative to the apparatus 10 with the couplingmember 72 being aligned with the inner channel 82 of the viscoelasticbushing 80 to avoid contact therebetween. Preferably, the center of thecoupling member 72 is aligned to coincide with the center of the innerchannel 82. As shown in FIG. 6, with the rails 46, 48 being in a loweredposition, there is no registration of the locator pin 96 and the locatoraperture 98. The hinge connection of the anti-skid arm 94 maintains thearm 94 in a spaced relationship relative to the bridge 70 with the arm94 being in a down position. In this manner, distortion of test resultsis avoided by ensuring application of force to the sled only through theinterengagement of the coupling member 72 and the coupling 76.

The second function of the anti-skid arm 94 is to ensure linear movementof the sled throughout the test run. As shown in FIG. 7, inwardlydirected guide pins 100 respectively extend from guide blocks 102. Theguide pins 100 are located to be spaced from the bridge 70 under normaloperation. However, non-linear translation of the sled 18 can be limiteddue to engagement of at least one of the guide pins 100 with the bridge70. The degree of spacing between the guide pins 100 and the bridge 70may be adjusted to increase or decrease the allowable range ofnon-linear translation of the sled 18.

As an additional feature, which can be used with any of theabove-described embodiments, the platen 16 may be provided with aheating mechanism 104. Referring to FIG. 1, the heating mechanism 104may be inset into the bottom surface of the platen 16. Although theheating mechanism 104 is only shown in the FIGURES with reference to thefirst embodiment, the heating mechanism 104 can be used with anyembodiment of the invention. The heating mechanism 104 can be anyconventional heating mechanism known by those skilled in the art. Theheating of the platen 16 during a test run will enable an operator tonot only determine the frictional characteristics of the interfacebetween the sample of sheet material 14 secured to the sled 18 and thetest medium or the test surface, but also the operator may observe theeffects of heat on the sample of sheet material 14.

Although preferred embodiments of the invention have been disclosed forillustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

We claim:
 1. An apparatus for testing static and kinetic frictionalcoefficients of the interface between a sample of a sheet material and atest medium, said apparatus comprising:a substantially horizontal platenfor accommodating the test medium; a sled for accommodating the sampleof sheet material; a pair of spaced, parallel rails for reversiblyraising and lowering said sled relative to said platen such that saidsled may be selectively placed on said platen with the sample of sheetmaterial being in engagement with the test medium, said sled beingformed for engagement with said rails; means for forcing translation ofsaid sled across at least a portion of the test medium and across atleast a portion of said platen; and load sensing means for measuring inreal time the forces required to initially cause translation of saidsled and for measuring in real time the forces required to maintaintranslation of said sled.
 2. An apparatus as claim 1, further comprisinga central processing unit for controlling said spaced, parallel railsand said forcing means, and said central processing unit also foracquiring data measured by said load sensing means and calculating thefrictional coefficients.
 3. An apparatus as in claim 2, furthercomprising a timing means for measuring time between placement of saidsled onto said platen by said spaced, parallel rails and actuation ofsaid forcing means.
 4. An apparatus as in claim 1, wherein said forcingmeans is configured to force said sled to translate relative to saidplaten at more than one rate of translation.
 5. An apparatus as in claim1, wherein the test medium includes a second sample of sheet material,and wherein said apparatus further comprises a clamping means for fixingsaid second sample of sheet material to said platen.
 6. An apparatus asin claim 5, wherein the test medium further includes a liquid, andwherein said apparatus further comprises a pan disposed on said platen,said pan dimensioned to accommodate said second sample of sheet materialand said liquid.
 7. An apparatus as in claim 1, wherein the test mediumis a liquid, and wherein said apparatus further comprises a pan disposedon said platen, said pan dimensioned to accommodate said liquid.
 8. Anapparatus as in claim 1, further comprising a heating means for heatingsaid platen.
 9. An apparatus for testing static and kinetic frictionalcoefficients of the interface between a sample of a sheet material and atest medium, said apparatus comprising:a substantially horizontal platenfor accommodating the test medium; a pair of spaced, parallel rails,said rails being formed to be simultaneously raised and lowered relativeto said platen; reversible motor means for reversibly raising andlowering said rails including during frictional testing; a sled foraccommodating the sample of sheet material, said sled including aplurality of wheels, said wheels being formed for engagement with saidrails; a drive arm for forcing said sled to translate in a single lineardirection relative to said platen; and load sensing means for measuringin real time the forces required to initially cause translation of saidsled relative to the test medium and said platen and for measuring inreal time the forces required to maintain translation of said sledrelative to the test medium and said platen, wherein reversible raisingand lowering of said rails relative to said platen with said wheels ofsaid sled engaging said rails allows for selective placement of saidsled on said platen with the sample of sheet material being inengagement with the test medium.
 10. An apparatus as in claim 9, furthercomprising a central processing unit for controlling said reversiblemotor means, said driving arm, and said load sensing means, and saidcentral processing unit also being for collecting data obtained by saidload sensing means and calculating the frictional coefficients.
 11. Anapparatus as in claim 10, further comprising a timing means formeasuring time between placement of said sled onto said platen andactuation of said drive arm.
 12. An apparatus as in claim 9, furthercomprising a semi-rigid coupling for connecting said sled to saiddriving arm.
 13. An apparatus as in claim 12, wherein said semi-rigidcoupling includes a viscoelectric bushing.
 14. An apparatus as in claim9, wherein said sled further comprises a plurality of axlescorresponding to said plurality of wheels, said axles extending from afirst side of said sled and a second side of said sled, said axlesextending from said first side all being formed with the same length,said axles extending from said second side all being formed with thesame length, wherein each said axle having a single said wheel beingmounted to one end thereof.
 15. An apparatus as in claim 14, whereinsaid axles are located on said sides of said sled such that with saidsled being disposed on said platen, said wheels are not in contact withsaid platen.
 16. An apparatus as in claim 14, wherein said axles arelocated on said sides of said sled such that with said wheels of saidsled engaging said rails and said rails being in a raised position, thesample of sheet material accommodated by said sled is disposed in agenerally parallel relationship relative to said platen.
 17. Anapparatus as in claim 9, wherein each said wheel is formed with at leastone circumferentially extending rim, and wherein with said wheelsengaging said rails, each said rim is in partial face-to-face engagementwith a single said rail.
 18. An apparatus as in claim 9, furthercomprising an anti-skid arm hingedly connected to said drive arm, saidanti-skid arm being formed with a pin extending therefrom, and whereinsaid sled being formed with an aperture dimensioned for registrationwith said pin of said anti-skid arm.
 19. An apparatus as in claim 18,wherein said anti-skid arm is formed with means for limiting non-lineartranslation of said sled.
 20. An apparatus as in claim 9, wherein thetest medium includes a second sample of sheet material, and wherein saidapparatus further comprises a clamping means for fixing said secondsample of sheet material to said platen.
 21. As apparatus as in claim20, wherein the test medium further includes a liquid, and wherein saidapparatus further comprises a pan disposed on said platen, said pandimensioned to accommodate said second sample of sheet material and saidliquid.
 22. An apparatus as in claim 9, wherein the test medium is aliquid, and wherein said apparatus further comprises a pan disposed onsaid platen, said pan dimensioned to accommodate said liquid.
 23. Anapparatus as in claim 9, further comprising a heating means for heatingsaid platen.
 24. An apparatus for testing static and kinetic frictionalcoefficients of the interface between a sample of a sheet material and atest surface, said apparatus comprising:a substantially horizontalplaten defining the test surface; a sled for accommodating the sample ofsheet material; a pair of spaced, parallel rails for reversibly raisingand lowering said sled relative to said platen such that said sled maybe selectively placed on said platen with the sample of sheet materialbeing in face-to-face engagement with the test surface said sled beingformed for engagement with said rails; means for forcing translation ofsaid sled across at least a portion of said platen; and load sensingmeans for measuring in real time the forces required to initially causetranslation of said sled and for measuring in real time the forcesrequired to maintain translation of said sled.
 25. An apparatus as inclaim 24, further comprising a central processing unit for controllingsaid spaced, parallel rails and said forcing means, and said centralprocessing unit also for acquiring data measured by said load sensingmeans and calculating the frictional coefficients.
 26. An apparatus asin claim 24, further comprising a timing means for measuring timebetween placement of said sled onto said platen by said spaced, parallelrails and actuation of said forcing means.
 27. An apparatus as in claim24, wherein said rails being formed to be simultaneously raised andlowered relative to said platen, reversible motor means for reversiblyraising and lowering said rails, and wherein said sled includes aplurality of wheels, said wheels being formed for engagement with saidrails, whereby engagement of said wheels with said rails allows forrepeatable alignment of said sled relative to said platen.
 28. Anapparatus as in claim 24, wherein said forcing means is configured toforce said sled to translate relative to said platen at more than onerate of translation.
 29. An apparatus as in claim 24, further comprisinga heating means for heating said platen.
 30. An apparatus for testingstatic and kinetic frictional coefficients of the interface between asample of a sheet material and a test surface, said apparatuscomprising;a substantially horizontal platen defining the test surface;a pair of spaced, parallel rails, said rails being formed to besimultaneously raised and lowered relative to said platen; reversiblemotor means for reversibly raising and lowering said rails; a sled foraccommodating the sample of sheet material; said sled including aplurality of wheels, said wheels being formed for engagement with saidrails; a drive arm for forcing said sled to translate in a single lineardirection relative to said platen; and load sensing means for measuringin real time the forces required to initially cause translation of saidsled relative to sais platen and for measuring in real time the forcesrequired to reversible raising and lowering of said rails relative tosaid platen with said wheels of said sled engaging said rails allows forselective placement of said sled on said platen with the sample of sheetmaterial being in engagement with the test surface.
 31. An apparatus asin claim 30, further comprising a central processing unit forcontrolling said reversible motor means, said drive arm, and said loadsensing means, and said central processing unit also being forcollecting data obtained by said load sensing means and calculating thefrictional coefficients.
 32. An apparatus as in claim 30, furthercomprising a timing means for measuring time between placement of saidsled onto said platen and actuation of said drive arm.
 33. An apparatusas in claim 30, wherein said drive arm is configured to force said sledto translate relative to said platen at more than one rate oftranslation.
 34. An apparatus as in claim 30, further comprising aheating means for heating said platen.
 35. An apparatus for testingstatic and kinetic frictional coefficients of the interface between asample of a sheet material and a test medium, said apparatuscomprising:a substantially horizontal platen for accommodating the testmedium; a pair of spaced, parallel rails formed to be raised and loweredrelative to said platen; reversible motor means for reversibly raisingand lowering said rails; a sled accommodating the sample of sheetmaterial, said sled being formed to be supportably engaged by said railsincluding during frictional testing; drive means for causing relativemovement between said sled and said platen; and load sensing means formeasuring in real time the forces required to initially cause movementof said sled relative to the test medium and said platen and formeasuring in real time the forces required to maintain movement of saissled relative to the test medium and said platen, wherein reversibleraising and lowering of said rails with said sled being supportablyengaged thereon allows for selective placement of said sled on saidplaten with the sample of sheet material being in engagement with thetest medium.
 36. An apparatus for tesing static and kinetic frictionalcoefficients of the interface between a sample of a sheet material and atest surface, said apparatus comprising:a substantially horizontalplaten defining the test surface; a pair of spaced, parallel rails beingformed to be raised and lowered relative to said platen; reversiblemotor meansf or reversibly raising and lowering said rails; a sled foraccommodating the sample of sheet material, said sled being formed to besupportably engaged by said rails including during frictional testing;drive means for causing relative movement between said sled and saidplaten; and load sensing means for measuring in real time the forcesrequired to initially cause movement of said sled relative to saidplaten and for measuring in real time the forces required to maintainmovement of siad sled relative to said platen, wherein reversibleraising and lowering of said rails relative to said platen with saidsled being supportably engaged thereon allows for selective placement ofsaid sled on said platen with the sample of sheet material being inengagement with test surface.